This invention relates to semiconductor wafer-based devices. More particularly the present invention relates to semiconductor-based sensors. The present invention is also related to humidity sensors. More particularly, the present invention is related to a semiconductor-based relative humidity sensor including integrated signal conditioning.
Solid-state semiconductor devices are found in most electronic components today. Semiconductor-based sensors, for example, are fabricated using semiconductor processes. Humidity sensors are but one class of semiconductor-based sensors finding many industrial applications. Modern manufacturing processes, for example, generally require measurement of moisture contents corresponding to dew points between xe2x88x9240xc2x0 C. and 180xc2x0 C., or a relative humidity between 1% and 100%. There is also a need for a durable, compact, efficient moisture detector that can be used effectively in these processes to measure very small moisture content in gaseous atmospheres.
Humidity can be measured by a number of techniques. In a semiconductor-based system, humidity can be measured based upon the reversible water absorption characteristics of polymeric materials. The absorption of water into a sensor structure causes a number of physical changes in the active polymer. These physical changes can be transduced into electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer. Two of the most common physical changes are the change in resistance and the change in dielectric constant, which can be respectively translated into a resistance change and a capacitance change. It has been found, however, that elements utilized as resistive components suffer from the disadvantage that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the elements necessary to make a resistance measurement. The result is erroneous readings, among other problems.
Elements constructed to approximate a pure capacitance avoid the disadvantages of the resistive elements. It is important in the construction of capacitive elements, however, to avoid the problems that can arise with certain constructions for such elements. In addition, there can also be inaccuracy incurred at high relative humidity values where high water content causes problems due to excessive stress and the resulting mechanical shifts in the components of the element. By making the component parts of the element thin, it has been found that the above-mentioned problems can be avoided and the capacitance type element can provide a fast, precise measurement of the relative humidity content over an extreme range of humidity as well as over an extreme range of temperature and pressure and other environmental variables.
Humidity sensing elements of the capacitance sensing type usually include a moisture-insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure along with a layer or coating of dielectric, highly moisture-sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and reaching equilibrium in a short period of time. Capacitive humidity sensors are typically made by depositing several layers of material on a substrate material.
Semiconductor-based humidity sensors are well known in the art. For example, U.S. Pat. No. 4,564,882, entitled xe2x80x9cHumidity sensing elementxe2x80x9d, issued Jan. 14, 1986, discloses a capacitance humidity sensing element. The sensing element structure includes a substrate and a set of interdigitated electrodes deposited on the substrate surface. A first water permeable polymer film is deposited over the electrodes and a conductive mesh is formed over the first polymer film. A second polymer film is deposited over the mesh, burying the mesh between the two polymer films. The spacing between openings of the mesh is less than the thickness of the second polymer film. The square of the sum of the two polymer film thicknesses is minimized so that the response time is minimized. The mesh conductivity is made greater than a minimum value so that the resistive component of the impedance is small compared with the capacitive impedance, and thus the device impedance is independent of any instabilities in the conductive mesh impedance.
U.S. Pat. No. 6,222,376 entitled xe2x80x9cCapacitive moisture detector and method of making the same,xe2x80x9d issued Apr. 24, 2001, to Tenney, discloses a multi-layer polymer RH sensor. The Tenney patent describes an improved capacitive moisture detector that includes a ceramic substrate with a plurality of layers of interdigitated electrodes and a plurality of interleaved moisture sensitive dielectric layers. Alternate electrodes are electrically coupled to provide two electrical contacts and a circuit representing multiple capacitors connected in parallel. According to an exemplary embodiment, six layers are provided, resulting in a structure presenting five parallel connected capacitors, thereby providing a total capacitance of the detector which is ten times that of previous detectors having the same footprint. According to a preferred embodiment of the Tenney invention, each dielectric layer is made relatively thin to effectively decrease the distance between the plates of each of the five capacitors. Since capacitance is inversely proportional to the distance between the dielectric layers, the thinness of the electrodes in the present invention also serves to increase capacitance. According to another preferred embodiment of the Tenney patent, a floating porous conductive film is provided over the sixth electrode to contain the field of the top capacitor and render the detector immune from the effects of surface contaminants.
U.S. Pat. No. 4,831,325, entitled xe2x80x9cCapacitance measuring circuitxe2x80x9d, issued May 16, 1989, to Watson, discloses yet another semiconductor-based sensor for measuring humidity. In the Watson patent, a variable capacitor, which may be a humidity-sensitive capacitor, and a fixed reference capacitor are connected at a node. The node is clamped at a reference potential during a first phase of a two-phase measuring cycle as the variable capacitor is charged to a fixed voltage and the fixed capacitor is charged to a feedback voltage. The node is unclamped during the second phase and the capacitors are connected in a series loop to allow a redistribution of the charge in the capacitors or force a reversal of that charge with a voltage source. The deviation of the node from its reference potential after charge redistribution occurs is used as input to a feedback circuit that integrates that deviation over a number of cycles until it provides a feedback voltage of magnitude sufficient to cause the node deviation to be reduced to zero. A second reference capacitor can be supplied to provide an offset. The capacitors are constructed by simultaneous deposition on a substrate of a first plate followed by a dielectric film and a second plate. The second plate of the variable capacitor is porous to admit water molecules and the second plate of the fixed capacitor is impervious to water. According to Watson, simultaneous deposition provides similar characteristics for the capacitors.
An electrical connection problem has been found to reside in some semiconductor humidity sensor designs as described in, but not limited to, the Watson patent. Failed electrical connection can result where a large step height exists between the conductive electrical plates and the electrical interconnects, particularly during thermal/humidity cycling due to the relatively high Temperature Coefficient of Expansion of the sensing medium (e.g., dielectric film, polyimide insulator) and the swelling and shrinking of the sensing medium. By this mechanism, changing humidity or temperature can disconnect the sensing capacitor system from the chip electronics, or at least change the capacitive value of the sensor arrangement.
The present inventors have found that there is a need for improved humidity sensor designs. In view of the above-identified shortcomings of previous devices, the present inventors have invented an improved relative humidity sensor. Accordingly, the present invention is described and presented as a means to address the shortcomings currently found in present humidity sensor devices.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole. Additional objects and advantages of the current invention will become apparent to one of ordinary skill in the art upon reading the specification.
It is an object of the invention to provide an improved capacitive moisture detector that is not adversely affected by surface contamination.
It is another object of the invention to provide an improved capacitive moisture detector that is sensitive and capable of detecting very small to very large quantities of moisture.
It is still another object of the invention to provide an improved capacitive moisture detector that is compact.
In accordance with another aspect of the present invention, an interlayer dielectric of silicon nitride is added in a sensor to protect aluminum metal interconnects.
In accordance with another aspect of the present invention, aluminum or metal interconnects can also be covered with gold to virtually eliminate the corrosion failures.
In accordance with another aspect of the present invention, a new capacitor configuration is incorporated into the humidity sensor that eliminates the weak mechanical connection between the thin platinum and the thick aluminum or metal interconnects.
In accordance with another feature of the present invention, the capacitive sensor structure of the present invention is incorporated into an integrated sensor system on silicon.
In accordance with another aspect of the present invention, a charge balance circuit can be incorporated into the integrated silicon sensor.
In accordance with another aspect of the present invention, the sensing capacitor can be constructed for minimization of stray capacitance, which adversely affects the charge balance circuit.
In accordance with yet another aspect of the present invention, the calibration method employs an area reduction technique for balancing capacitance in a sensor structure, which minimizes error due to unbalanced capacitance.
In accordance with another aspect of the present invention, an integrated relative humidity sensor (IRHS) can include a planar humidity sensitive capacitor structure that includes: a thin porous platinum top plate, a humidity sensitive polyimide dielectric, and a metal (e.g., titanium-tungsten) bottom plate. Two capacitors can be effectively wired in series such that the metal bottom plates form the independent, electrically driven connections, and the thin platinum top layer is used to form the top plates and the floating series interconnection. The floating series interconnection thus eliminates problems associated with interconnection between the prior art thin platinum plate to thick aluminum or metal interconnections.
In accordance with another aspect of the present invention, the charge balance circuit can generally reject stray capacitance from the bottom plate of each capacitor to the underlying substrate.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.